A mono-tube type hydraulic shock absorber is generally configured to include the following: a cylinder, a free piston that is slidably inserted into the cylinder to partition the inside of the cylinder into a liquid chamber and a gas chamber, a piston that is slidably inserted into the cylinder to partition the liquid chamber into an extension-side chamber and a compression-side chamber, and a piston rod that is connected at one end to the piston and is movably inserted into the cylinder. Such a mono-tube type hydraulic shock absorber is used by, for example, interposing it between a vehicle body and a vehicle axle of an automobile.
In the above-described mono-tube type hydraulic shock absorber, the piston rod moves into and out of the cylinder during extension and compression. Therefore, the free piston displaces within the cylinder to change the capacity of the gas chamber, and this compensates for the volume of the piston rod moving into and out of the cylinder.
An annular groove is formed on the outer periphery of the free piston, and an O ring is fitted therein. In the above-described mono-tube type hydraulic shock absorber, the O ring seals between the cylinder and the free piston, and thereby gas within the gas chamber can be prevented from leaking into the liquid chamber (for example, refer to JP8-152037A).